再生水灌溉区农田包气带及地下水中硝酸盐分布特征及来源研究

Distribution characteristics and sources of nitrate in the unsaturated zone and groundwater of farmlands in an area irrigated with reclaimed water

本文通过对华北平原典型再生水灌溉区(河北省石家庄洨河流域)的包气带土壤、地表水和地下水进行采样分析,对硝酸盐在多种环境介质中的来源与环境行为进行了研究,识别了再生水灌溉区地下水硝酸盐污染来源,明确了不同灌溉条件对包气带土壤中硝酸盐迁移的影响。在受到城市再生水严重影响的洨河流域,地下水中的硝酸盐浓度分布范围在4.0 mg·L^-1到156.6 mg·L^-1之间,已经形成了距离河道2 km、深度70 m的硝酸盐高值区域,经过计算硝酸盐的垂向扩散速率为每年1~2 m。硝酸盐与氯离子的相关性表明,城市再生水是再生水灌溉区包气带、地表水和地下水中硝酸盐的主要来源。利用Geoprobe获取利用不同灌溉水农田土壤剖面样品,研究再生水对厚包气带NO3^-N垂向分布影响,再生水灌溉区和地下水灌溉区中包气带土壤的NO3?-N的平均含量为137.0 mg·kg^-1和107.7 mg·kg^-1,最高含量523.2 mg·L^-1和725.9 mg·L^-1,分别出现1.20 m和0.85 m深度,分布规律有着明显的差别。包气带土壤硝酸盐与氯离子的相关性分析表明,再生水灌溉区土壤硝酸盐主要来源于城市再生水,而地下水灌溉区可能来源于农田氮肥。地下水年龄和硝酸盐之间关系表明,地下水中1975年以前补给的硝酸盐浓度低于1975年以后补给,地下水硝酸盐污染与包气带氮入渗的历史过程密切相关。在华北平原特殊的地质水文背景下,农田面源污染对地下水的影响有限,但再生水灌溉区地下水硝酸盐污染的风险较高。

The present study aimed to presenting the source and environmental behavior of nitrate in various environmental media by sampling and analyzing the soil in the unsaturated zone, surface water, and groundwater of a typical area irrigated with reclaimed water in the North China Plain. This study also identified the sources of groundwater nitrate pollution in the area, and determined the effects of different irrigation conditions on nitrate migration in the soil in the unsaturated zone. In the Xiao River basin, which is seriously affected by reclaimed urban water, nitrate concentrations in groundwater ranged from 4.0 mg·L^-1 to 156.6 mg·L^-1, nitrate in the groundwater had formed a high-value area of 2 km from the river channel and 70 m in depth. And nitrate moved down 1-2 m every year. The correlation between nitrate and chloride ion indicated that reclaimed urban water was the main source of nitrate in the unsaturated zone, surface water, and groundwater of the area irrigated with reclaimed water. Geoprobe system was used to obtain continuous soil core samples from the typical unsaturated zones to study the influence of reclaimed water on the vertical distribution of NO3--N in thick unsaturated zone. The average contents of NO3--N in the reclaimed water and groundwater irrigation areas were 137.0 mg·kg^-1 and 107.7 mg·kg^-1, with the peak values of 523.2 mg·kg^-1 at a soil depth of 1.20 m and 725.9 mg·kg^-1 at a depth of 0.85 m, respectively. The correlation analysis between soil nitrate and chloride ion indicated that the NO3--N in the reclaimed water irrigation area was mainly affected by reclaimed urban water, whereas in the groundwater irrigation area it may because the use of agricultural nitrogen fertilizers. By comprehensively analyzing the relationship between groundwater age and nitric acid, the historical process of the nitrogen source causing the groundwater nitrate pollution could be reproduced on a time scale. Under the unique hydrogeological background of the North China Plain, the nonpoint source pollution of the farmland has a limited effect on groundwater pollution, while the risk of groundwater nitrate nitrogen pollution is relatively high around the reclaimed water river.